Removal of submicron particles from gas streams is often critical for ensuring compliance with emission limits for many industrial processes, such as urea finishing. For instance off-gas from urea prilling towers contains a relatively large amount and/or large fraction of submicron particles, for instance compared to the off-gas from a urea fluid bed granulator. Hence, the removal of submicron urea particles is particularly important in order to meet ever more stringent regulations and limits on the emission of urea. The removal of ammonia from urea finishing off-gas is also necessary. Background references relating to the removal of urea dust from off-gas from a urea finishing section include WO 2015/002535 and WO 2015/072854.
It may be observed that particle size distribution of urea prilling tower off-gas has a peak between 0.1 μm and 1 μm aerodynamic particle size, with a cumulative mass of for example about 70 mg/Nm3 provided by particles <10 μm (about 50 wt. % total particulate matter, PM). Off-gas from urea granulation may for example contain about 25 mg/Nm3 of particles <10 μm. For compliance with current and future emission limits, significant removal of submicron particles, e.g. urea dust, is essential. Available particle capture technologies generally have very low efficiency for submicron particles or have a large pressure drop.
In urea prilling, urea melt is supplied at the top of a prilling tower, and distributed as droplets. The urea melt droplets solidify as they fall down while cooling against a large quantity of upward-moving air. Urea prills are withdrawn from the bottom. The fresh cooling air enters the bottom of the prilling tower. The off-gas comprising urea and ammonia leaves the prilling tower near the top.
A prilling tower can for instance have a height of for example 60 m to 80 m. Smaller plants may have a free fall path of 50 m or less. Some of the largest plants have prilling towers of 125 m height. Emissions can for example be 0.5 to 2.5 kg urea dust per ton urea prills (35 to 125 mg/Nm3) and about 0.5 to 2.7 kg NH3 per ton (35-245 mg/Nm3). Urea dust emissions of more than 200 mg/Nm3 have been reported for some existing urea prilling towers. An example indicative air flow for a urea prilling tower is 500 000 Nm3/hr. A larger urea prilling tower may for instance have 900 000 Nm3/hr, with a urea capacity of 75-100 mt/hr (metric ton per hour).
Older prilling towers frequently vent off-gas directly to air without any urea or ammonia abatement. The tower construction generally sets a maximum for the weight for the design of any abatement systems installed on top as part of revamping. The off-gas from some prilling towers has a low pressure drop, in particular the off-gas from prilling towers operating with natural draft. Existing emission abatement technologies typically would require large blowers and fans to maintain sufficient pressure drop, since generally submicron particle removal requires high pressure drop. Current systems are hence not suitable for installation on top of an existing prilling tower. The possibility of first bringing the off-gas to lower elevation through a duct would introduce an additional significant pressure drop. In view of the large airflows this would lead to a significant increase of the power consumption. The construction of a duct from the top to the bottom of the urea prilling tower is also challenging and expensive, and introduces the risk of plugging in the duct between the prilling tower and the emission abatement system.
For currently available emission abatement technologies, dust scrubbers, especially if combined with an acid washer to reduce ammonia emissions, are generally considered to be feasible only for forced draft prilling towers where air fans are available, and not for natural draft urea prilling towers. For instance U.S. Pat. No. 4,424,072 to Lerner shows in FIG. 1 an apparatus comprising vertical urea prilling tower 11 with a plurality of scrubbers 17 provided above the top of the tower. This patent teaches that the apparatus includes facilities for injecting a stream of air into the lower part of the prilling tower, accomplished by forced draft, induced draft, or combination thereof. As used in the art, induced draft towers use a centrally located fan at the top and forced draft cooling towers use a fan located near the bottom.
Accordingly, there is a need for more effective emission abatement systems and methods that can operate with a low pressure drop and can effectively remove submicron particles from gas streams. More in particular, there is a need for better urea and ammonia emission abatement systems and methods for urea prilling towers.